Semver instead of revisions for charms?

[Mark Shuttleworth]

The approach we are discussing in the snappy world is to use "gateway"
revisions, which are both forward and backward compatible, which encode
the transition logic. They essentially handle the difficult upgrade, and
enable subsequent revisions to drop support for the older schemes.
Upgrades would then be guaranteed to go through those gateway revisions
(i.e. from rev-current to rev-stable via a series of rev-gateways). If
this sounds useful we could plan for the same approach in charms.

Mark